Any kind of possibility to access the supply plumbing elsewhere in the house such as the crawlspace or a different room that you are willing to cut out some drywall?

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Nope. The knock best I can tell is right near the bathrooms, and the only access to that plumbing is through tile or the roof, both of which would cost a lot of money to fix. It's not a serious knock right now, sounds like the pipe might be moving somewhere in the neighborhood of 1/32-1/16", but nonetheless it is a weak point.

Taking into account your anal nature it suprizes me that the water gods have placed you in the barrel on your own home.

Since i know you followed the crews and anything that didn't pass the white glove test was addressed personally, i too am dazed and confused.

Could you have a leak in the main before the house? What about backflow? Is the supply copper or pex?
pex moves a lot, even when strapped firmly.
Could debris be in the line?

Have you placed gauges on spigots and bibs to measure pressure when flushing, done a meter check or purchased a buttload of beer? I have a feeling a lot of brewskies will be consumed before this puzzle is solved.